Regions significantly more activatedin patients than control subjects. Talairach coordinates are shown in Table 3. For regions significantly moreactivated in controls, see "Results" section and Table 3.

ContextObsessive-compulsive disorder (OCD) is clinically heterogeneous, yet
most previous functional neuroimaging studies grouped together patients with
mixed symptoms, thus potentially reducing the power and obscuring the findings
of such studies.

InterventionAll subjects participated in 4 functional magnetic resonance imaging
experiments. They were scanned while viewing alternating blocks of emotional
(washing-related, checking-related, hoarding-related, or aversive, symptom-unrelated)
and neutral pictures, and imagining scenarios related to the content of each
picture type.

Main Outcome MeasureBlood oxygenation level–dependent response.

ResultsBoth patients and control subjects experienced increased subjective
anxiety during symptom provocation (patients significantly more so) and activated
neural regions previously linked to OCD. Analyses of covariance, controlling
for depression, showed a distinct pattern of activation associated with each
symptom dimension. Patients demonstrated significantly greater activation
than controls in bilateral ventromedial prefrontal regions and right caudate
nucleus (washing); putamen/globus pallidus, thalamus, and dorsal cortical
areas (checking); left precentral gyrus and right orbitofrontal cortex (hoarding);
and left occipitotemporal regions (aversive, symptom-unrelated). These results
were further supported by correlation analyses within patients, which showed
highly specific positive associations between subjective anxiety, questionnaire
scores, and neural response in each experiment. There were no consistently
significant differences between patients with (n = 9) and without (n = 7)
comorbid diagnoses.

ConclusionsThe findings suggest that different obsessive-compulsive symptom dimensions
are mediated by relatively distinct components of frontostriatothalamic circuits
implicated in cognitive and emotion processing. Obsessive-compulsive disorder
may be best conceptualized as a spectrum of multiple, potentially overlapping
syndromes rather than a unitary nosologic entity.

Despite this heterogeneity, most previous neuroimaging studies of OCDhave grouped together patients with mixed symptoms,18- 23 thuspotentially reducing their power and obscuring their findings. Few studieshave examined the neural correlates of different symptom dimensions. Rauchet al24 found that checking symptoms correlatedwith increased, and symmetry/ordering with reduced, regional cerebral bloodflow in the striatum, while washing symptoms correlated with increased regionalcerebral blood flow in bilateral anterior cingulate and left orbitofrontalcortex. Using functional magnetic resonance imaging (fMRI), Phillips et al25 compared OCD patients with mainly washing or checkingsymptoms while they viewed generally aversive or washing-related pictures.When viewing washing-related pictures, only washers demonstrated activationsin regions implicated in emotion and disgust perception, ie, visual regionsand the anterior insula26- 28;checkers demonstrated activations in frontostriatal regions and the thalamus.In another fMRI study, patients with OCD with predominantly washing symptomsdemonstrated greater activation than controls in the right insula, ventrolateralprefrontal cortex, and parahippocampal gyrus when viewing disgust-inducingpictures.29 Limitations of these studies includedthe artificial division between washers and checkers and the exclusive useof washing-related material, but taken together, they suggest that differentsymptoms may be mediated by distinct neural systems and that previous discrepantfindings may result from phenotypic variations in the studied samples.

Building on recent pilot work from our group,30 weused a symptom provocation paradigm to examine, within thesame patients, the neural correlates of washing, checking, and hoardingsymptom dimensions of OCD. This dimensional approach is methodologically superiorto categorically dividing patients into mutually exclusive subgroups becausemonosymptomatic patients are infrequent and such a division is therefore artificial.On the basis of previous studies, we hypothesized that (1) anxiety would beprovoked in response to all types of emotional material both in patients andin controls but more so in patients22,30,31;(2) symptom provocation would activate regions previously implicated in OCD,18- 23 bothin patients and in controls,30 but more soin patients; and (3) distinct patterns of neural response would be associatedwith the provocation of each symptom type. Specifically, in patients comparedwith controls, the provocation of (a) washing-relatedanxiety would predominantly activate areas involved in emotion and disgustperception, ie, ventromedial prefrontal and paralimbic regions24,25,29,30;(b) checking-related anxiety, regions involved inattentional and motor functions, ie, dorsolateral prefrontal cortex, thalamus,and striatal regions24,25,30;and (c) hoarding-related anxiety, ventromedial prefrontaland paralimbic regions.30 Finally, we expectedthat, in patients, the magnitude of activation in the predicted regions withineach experiment would be significantly correlated with the corresponding subjectiveanxiety and/or symptom dimension scores.

METHODS

SUBJECTS

Seventeen patients with OCD (11 inpatients, 6 outpatients) who wereat various stages of treatment were recruited from 2 specialized cognitivebehavioral therapy clinics in London, England. This was a consecutive sample,but an effort was made to ensure that washing, checking, and hoarding symptomswere sufficiently represented. One outpatient reported having closed his eyesin the scanner and was excluded. Axis I and II diagnoses were made accordingto DSM-IV.32,33 Patientswith comorbid diagnoses were not excluded provided that OCD was the main problemfor which treatment was sought. Exclusion criteria were brain injury, anyneurologic condition, psychosis, and substance abuse.

Seventeen healthy volunteers of similar demographic characteristicswere recruited among ancillary staff at the Institute of Psychiatry. Theyreported no history of neurologic or psychiatric disorder and were unmedicated.Data from 10 of these control subjects were partially reported elsewhere.30 The Ethics Committee of the Maudsley Hospital/Instituteof Psychiatry, London, approved the study protocol, and all subjects signedan informed consent form before their participation.

MEASURES

In the OCD group, severity and types of OCD symptoms were assessed withthe Yale-Brown Obsessive-Compulsive Scale and the Symptom Checklist.34,35

Depression was assessed with the Beck Depression Inventory (BDI).44 The state subscale of the State-Trait Anxiety Inventory45 was administered immediately before the scan.

STIMULI

Fifty color pictures of scenes rated as aversive or disgusting by normalsubjects (eg, insects, mutilated bodies, decaying food) and 50 pictures ofneutral scenes (eg, furniture, nature scenes, household items) were selectedfrom a standard set of stimuli.46 These stimuliwere carefully chosen to avoid resembling common triggers of OCD symptoms.In addition, pictures depicting contamination/washing, aggressive/checking,and hoarding material (50 of each type) were obtained with a standard digitalcamera. For each symptom type, 3 clinicians with experience in OCD had previouslylisted the most common items that were provocative of anxiety and the urgeto ritualize in patients with OCD. Examples of the pictures are public telephoneor toilet, money, syringe, and ashtray (washing); electric appliances, stove,open door, and purse (checking); and old newspapers or magazines, old clothesor toys, empty bottles or cans, and trash bins (hoarding).

A total of 250 scenes were selected after an independent group of 9normal volunteers (unrelated to the study) had rated an originally largerpool of pictures according to their level of visual complexity, anxiety, anddisgust on a 0 to 3 scale (0, none; 3, high). Pictures that were too simpleor too complex were excluded, and an effort was made to avoid using washing-relatedpictures that could be perceived as very aversive by normal individuals. Thefinal 250 stimuli were well matched regarding visual complexity and, as intended,the normally aversive or disgusting pictures induced more anxiety and disgustthan the other 3 types of pictures (data not shown).

SYMPTOM PROVOCATION PARADIGM

All subjects participated in four 6-minute experiments in which theyviewed ten 20-second alternating blocks of emotional (washing, checking, orhoarding related or normally aversive) and neutral pictures. The order inwhich the 4 experiments were conducted was fully counterbalanced, as was theorder of the emotional and neutral conditions within each experiment. Moredetails can be found in Mataix-Cols et al.30

Before the presentation of each set of pictures, subjects were playeda prerecorded voice file by means of high-fidelity pneumatic headphones, instructingthem to imagine being in a particular situation while looking at the scenesthey were about to see. Examples of these instructions are as follows: "Imaginethat you must come into contact with what's shown in the following pictureswithout washing yourself afterwards" (washing); "Imagine that you are notsure whether you switched off or locked the following objects and it is impossiblefor you to go back and check" (checking); "Imagine that the following objectsbelong to you and that you must throw them away forever" (hoarding); "Imaginethat you must touch or stand by the following objects" (aversive); "Imaginethat you are completely relaxed while looking at the following scenes" (neutral).

After each set of pictures, another prerecorded sound file of the question"How anxious do you feel?" was played and the subjects rated their subjectiveanxiety on a 0 (no anxiety) to 8 (extreme anxiety) scale.

In each 20-second stimulus presentation block, subjects viewed either10 provocative or 10 neutral pictures. Each picture was presented for 1950milliseconds, with an interstimulus interval of 50 milliseconds. Ten whole-brainvolumes were acquired during each stimulus presentation block. Each stimulusblock was followed by (1) an 8-second period of complete silence during whichsubjects were asked to rate their level of anxiety and (2) an additional 8-secondperiod during which the subjects listened to a sound file containing instructionspertinent to the next stimulus block. Four "dummy volumes" were excited duringthis 8-second period by means of exactly the same radiofrequency envelopeand gradient section selection parameter, with the same repetition time of2 seconds to allow the magnetization to reach an equilibrium amplitude beforethe next period of data acquisition. The frequency-encoding gradient was turnedoff during this period to minimize acoustic noise and ensure that the instructionswere heard clearly by the subjects.48 The 4dummy volumes were later discarded from the time series.

Data were analyzed with software developed at the Institute of Psychiatry,using a nonparametric approach. Data were first realigned49 tominimize motion-related artifacts and smoothed by means of a gaussian filter(full width at half maximum, 7.2 mm). Responses to the experimental paradigmswere then detected by time-series analysis using gamma variate functions (peakresponses weighted between 4 and 8 seconds) convolved with the experimentaldesign to model the blood oxygen level–dependent response. A goodness-of-fitstatistic and a measure of the mean power of neural response (the sum of squares[SSQ] ratio) was computed at each voxel. This was the ratio of the sum ofsquares of deviations from the mean intensity value due to the model (fittedtime series) divided by the sum of squares due to the residuals (originaltime series minus model time series). To sample the distribution of SSQ ratiounder the null hypothesis that observed values of SSQ ratio were not determinedby experimental design (with minimal assumptions), the time series at eachvoxel was permuted by a wavelet-based resampling method.50,51 Thisprocess was repeated 10 times at each voxel to produce the distribution ofSSQ ratios under the null hypothesis. Voxels activated at any desired levelof type I error can then be determined by obtaining the appropriate criticalvalue of SSQ ratio from the null distribution. Individual brain activationmaps were produced for each subject for each experiment vs the neutral condition.

Group Maps

To extend inference to the group level, the observed and randomizedSSQ ratio maps were transformed into standard space52 bya 2-stage process53 using spatial transformationscomputed for each subject's high-resolution structural scan. Once the statisticmaps were in standard space, a generic brain activation map was produced foreach experimental condition by testing the median observed SSQ ratio overall subjects at each voxel in standard space (median values were used to minimizeoutlier effects), against a critical value of the permutation distributionfor median SSQ ratio ascertained from the spatially transformed wavelet-permuteddata.53 For greater sensitivity and to reducethe multiple comparison problem encountered in fMRI, hypothesis testing wascarried out at the cluster level using methods developed by Bullmore et al.54 This method estimates the probability of occurrenceof clusters under the null hypothesis using the distribution of median SSQratios computed from spatially transformed data obtained from wavelet permutationof the time series at each voxel (see preceding section). Imagewise expectationof the number of false-positive clusters under the null hypothesis is setfor each analysis at less than 1.

Between-Group Differences (Analysis of Covariance)

Analysis of covariance was carried out on the SSQ ratio maps in standardspace by first computing the difference in mean SSQ ratio between groups ateach voxel. The BDI scores were used as covariates in all analyses. Subsequentinference of the probability of this difference under the null hypothesiswas made by reference to the null distribution obtained by repeated randompermutation of group membership and recomputation of the mean difference inSSQ ratio. Cluster-level maps were then obtained as described by Bullmoreet al.54 We set a voxelwise P value of .025 and a clusterwise P valueof .0001. This method ensured a total number of false positives close to zero.Correction for multiple comparisons was not required, as thresholds were seton an image-wide basis, not a voxelwise basis.

Partial Correlation Analyses

Correlation of fMRI blood oxygen level–dependent responses withbehavioral measures was determined by first computing the Pearson productmoment correlation coefficient at each voxel between the standardized powerof the fMRI response (SSQ ratio) and the behavioral variable for each subject.The null distribution of correlation coefficients was then computed by randomlypermuting group membership (see previous section) and recomputing the correlationcoefficient in an analogous fashion to that used for computation of groupdifferences. Cluster level maps of significant correlations were then computedas described by Bullmore et al.54 The voxelwiseand clusterwise P values were set at .05 and .0001,respectively, ensuring less than 1 false positive. For each of the significantclusters identified by the above method, we next extracted the SSQ ratio ofeach participant and conducted a series of partial correlation analyses withthe relevant anxiety and questionnaire measures, controlling for BDI scores.

RESULTS

There were no statistically significant differences between patientsand controls on any demographic variable, but patients had more severe obsessive-compulsive(PI-R, SI-R) and depressive (BDI) symptoms (Table 1). Scores on the PI-R and SI-R suggested marginal levelsof obsessive-compulsive symptoms in the control group. Patients and controlsexperienced similar moderate state anxiety levels (State-Trait Anxiety Inventory)in anticipation of having a scan. All patients endorsed more than 1 symptomtype (Table 2).

SUBJECTIVE ANXIETY RATINGS

Mixed-model analyses of variance with group (patient vs control) asbetween-groups factor and experimental condition (emotional vs neutral) aswithin-subjects factor showed significant main group and condition effectsin all 4 experiments, indicating that the paradigm was effective in provokinganxiety and that patients with OCD experienced higher anxiety levels thancontrols. The group × condition interaction effect was also significantfor the washing and hoarding experiments, suggesting that the difference betweenthe emotional and neutral conditions was greater in the OCD than in the controlgroup (Figure 1).

Correlation analyses within the control group showed few significantassociations between subjective anxiety or clinical scales and brain activation.Positive correlations were found in right occipitotemporal regions (BA19/18/37)in all experiments. Negative correlations were found in left occipital cortex(BA31/19/18; washing and checking experiments), right precentral/inferiorfrontal gyrus (BA6/44; washing experiment), and right middle frontal gyrus(BA46/9; checking experiment).

To our knowledge, this was the first symptom-provocation study to examinethe neural correlates of different symptom dimensions of OCD in a representativesample of multisymptomatic patients using a dimensional approach. The mainfinding was that washing, checking, and hoarding symptom dimensions of OCDwere mediated by distinct but partially overlapping neural systems.

In the checking experiment, patients showed greater activation thancontrols predominantly in regions important for motor and attentional functions:a large bilateral cluster in the subthalamic region including various brainstemnuclei, right putamen/globus pallidus, right thalamus, and various dorsolateralcortical regions (inferior frontal, dorsal anterior cingulate, medial/superiorfrontal, middle/medial frontal, and precentral gyri). There were fewer statisticallysignificant differences in emotion-processing regions (right hippocampus anda small bilateral cluster in the subgenual anterior cingulate gyrus, extendingto the orbitofrontal cortex). Correlation analyses showed positive correlationsbetween subjective anxiety and activation in left precentral/superior frontal(BA6), left inferior frontal gyrus (BA44), bilateral putamen/globus pallidus,and left thalamus during this experiment. Correlations with PI-R checkingscores gave similar findings: positive correlations in bilateral globus pallidus/putamenand left thalamus. Thus, both state and trait checking-related anxiety correlatedwith activation in similar regions.

These findings are consistent with those of Rauch et al,24 whofound positive correlations between scores on a checking scale and regionalcerebral blood flow in bilateral striatum, and Phillips et al,25 whofound that only checkers activated dorsal prefrontal (anterior cingulate andinferior frontal gyrus) and visual regions, thalamus, and caudate nucleus.Hypermetabolism in the putamen has been inconsistently reported in OCD.61,62 It is possible that an excess ofpatients with checking symptoms were recruited for these studies but few describedtheir samples in detail. A majority of patients (7 of 11) in the Perani etal61 positron emission tomographic study werelabeled as "checkers." These authors found increased metabolic rates in thecingulate gyrus, thalamus, and putamen/globus pallidus but no orbitofrontalor caudate involvement, findings similar to ours. We suggest that the provocationof checking-related anxiety (or the suppression of checking rituals) is associatedwith dysfunction in a circuit that is important for attentional and motorfunctions as well as the inhibition of unwanted impulses59,63- 65 ratherthan emotion processing per se.

In the hoarding experiment, patients showed increased activation inleft precentral/superior frontal (BA4/6), fusiform (BA37), and right orbitofrontal(BA11) gyri, compared with controls. Significant correlations with hoarding-relatedanxiety were found in left precentral/superior frontal gyrus (BA4/6). An associationbetween hoarding-related anxiety and activation in motor cortex was unpredicted,and its significance is uncertain. Increased activation in right orbitofrontalcortex is congruent with the intense emotional reactions these patients experiencewhen they are asked to discard their possessions.66 Activityin this region has been shown to be negatively correlated with response topharmacotherapy55,67- 69 andpositively correlated with response to cognitive behavioral therapy.68 The relationship between our finding of increasedactivation in this region during the provocation of hoarding symptoms andthe well-documented lack of treatment response of these patients3,5,14,15,17 requiresinvestigation.

The inclusion of an aversive, symptom-unrelated experiment allowed usto explore the neural correlates of general emotional reactivity independentof the content of the patients' symptoms. Although patients experienced moresubjective anxiety than controls, they showed greater activation only in occipitotemporalregions, suggesting that the findings of the foregoing experiments were mostlysymptom specific.

As in previous symptom provocation studies,22,30,31 thepresentation of OCD symptom–like material was associated with significantincreases in subjective anxiety not only in patients but also in controls.Consistent with a few previous reports,25,30,31 controlsactivated brain regions similar to those activated by patients. These resultsare not surprising, as these areas have been repeatedly associated with theinduction of various emotional states in normal subjects70- 75 andpatients with other anxiety disorders.76- 78 Greateractivation in these regions among patients paralleled their higher anxiety,reflecting the greater salience of the provoked stimuli in the patient group.

Controls showed greater activation than patients in bilateral visualareas in all experiments. Furthermore, significant correlations with subjectiveanxiety and symptom rating scores were observed primarily in occipital regions.Increased activation within visual cortex was repeatedly demonstrated in responseto emotive compared with neutral visual stimuli in healthy individuals.30,79,80 It is plausible thatcontrols directed their attentional resources to the processing of the pictures'visual details rather than their emotional salience.25,81

Controls showed greater activation than patients in left inferior prefrontalregions during the washing (BA47) and checking (BA44/6) experiments. Similarregions have been associated with suppression of negative emotions82 and might reflect more successful regulation of anxietyin controls. During the aversive experiment, controls also showed greateractivation than patients in the left insula (extending to the ventrolateralprefrontal and superior temporal cortices), which are emotion and disgustperception areas. This might reflect a bias toward highly aversive (but notsymptom-related) material in controls and the opposite pattern in patients.

This study did have certain limitations. We did not exclude patientswith comorbidity. Comorbid depression has been found to affect resting-stateregional glucose metabolism in positron emission tomographic studies.83- 85 However, comorbidityhad little impact on our results: (1) it was constant across the 4 experiments;(2) patients with (n = 9) and without (n = 7) comorbidity had similar sociodemographicand clinical characteristics and showed no consistent differences in brainactivity; (3) BDI scores were used as covariates in all analyses; and (4)there were few consistent differences between patients and controls in theaversive control experiment.

Since most patients (n = 12 [75%]) were taking medications, we couldnot compare medicated and unmedicated patients. However, (1) medications wereconstant across the 4 experiments; (2) several studies have demonstrated thatdrug treatment has a normalizing effect on pretreatment functional abnormalities85,86; medication would therefore haveattenuated rather than inflated our results; (3) symptom provocation studieswith22 or without23 medicatedpatients reported similar results; and (4) there were no consistent differencesbetween patients and controls in the aversive control experiment.

The sample was relatively small, but this is the largest fMRI studyin OCD to date. The reported effects were strong and consistent across variousmethods of analysis. It is possible that our hoarding experiment was underpowered,since only half of our sample had current hoarding symptoms; further researchon the hoarding dimension is warranted. The neural correlates of the symmetry/orderingdimension remain to be investigated.

CONCLUSIONS

The relative inconsistency of findings from previous functional neuroimagingstudies of OCD may have resulted from phenotypic variations among subjectgroups. Replication of our findings would suggest that discrete neural systemsmight mediate the expression of different symptoms. Because of the neuroanatomicproximity within the frontostriatothalamic loops,59 itis not surprising that the different symptom dimensions often coexist in anygiven patient. Obsessive-compulsive disorder could be better understood asa spectrum of multiple potentially overlapping syndromes that are likely tobe continuous with "normal" worries and extend beyond the traditional nosologicboundaries of OCD. Each symptom dimension might reflect the dysregulationof highly conserved complex and partially overlapping neural systems thatserve to detect, appraise, and respond to potential threats.87